Electrical transmission and distribution networks consist of a staggering number of transformers, capacitor banks, reactors, motors, generators and other major pieces of electrical equipment. Such equipment is extremely expensive. Further, each piece of such equipment typically plays a vital role in the distribution of power to end users, such that an outage caused by the equipment being damaged or taken out of service for repair or replacement, may have extremely costly consequences. As airesult, such equipment is typically protected from potentially damaging overvoltages and overcurrents by protective components, such as fuses and surge arresters.
A fuse is a current interrupting device which protects a circuit by means of a current-responsive fusible element. When an overcurrent or short-circuit current of a predetermined magnitude and duration is conducted through the fuse, the fusible element melts, thereby opening the circuit. After having interrupted an overcurrent, the fuse must be located and replaced in order to restore service.
A type of fuse that is particularly desirable in today's power distribution networks is the current limiting fuse. The current-limiting fuse has at least three features that have made it extremely desirable for use by the utilities:
(1) Interruption of overcurrents is accomplished quickly and without the violent expulsion of flaming arc products or gases or the development of forces external to the fuse body, all of which are characteristic of expulsion type fuses. This enables the current-limiting fuse to be used indoors, or even in small enclosures. Furthermore, since there is no discharge of hot gases or flame, only normal electrical clearances from other apparatus need to be provided. PA1 (2) A current-limiting action or reduction of current through the fuse to a value less than that otherwise available from the power-distribution network at the fuse location occurs if the overcurrent greatly exceeds the continuous-current rating of the fuse. Such a current reduction reduces the stresses and possible damage to the circuit up to the fault or to the faulted equipment itself, and also reduces the shock to the distribution network. PA1 (3) Very high interrupting ratings are achieved by virtue of its current-limiting action so that current-limiting fuses can be applied on medium or high-voltage distribution circuits of very high available short-circuit currents.
A current limiting fuse typically consists of one or more fusible elements of silver wire or ribbon of a required length which are electrically connected at their ends to a pair of electrical terminations. The subassembly--consisting of the fusible element and end terminations--is placed in a tubular housing that is made of a highly temperature-resistant material, and the housing is then typically filled with high-purity silica sand and sealed. Terminals on the ends of the housing interconnect the fuse with the distribution network. The entire assembly is generally referred to as a current limiting fuse.
Another important consideration to utilities in fuse selection relates to the ability of the fuse to be physically integrated within the utilities' existing network, and the ease and cost of installation and service. In present-day networks, fuseholders are typically installed in mountings which are known as "cutouts." Generally speaking, a cutout consists of a mounting having an insulating support designed to be mounted on a utility pole or crossarm and having a pair of spaced-apart terminals which are designed to receive and electrically engage a fuseholder, a switch assembly, or a combination thereof. When installed, the fuseholder or switch bridges the "gate" between the terminals of the cutout mounting.
The term "fuse cutout" usually refers to the combination of a cutout mounting, as described above, with a fuseholder. The fuseholder that is most typically employed in a fuse cutout is designed to be easily disengaged from the terminals of the cutout to permit quick and convenient fuse removal and replacement.
Present day fuse cutouts offer a relatively convenient and low cost means of fusing, and thereby protecting electrical distribution systems. Further, the industry is adopting a dimensional standard for expulsion fuseholders and cutout mountings, such that an expulsion fuseholder from one manufacturer will properly fit into the mounting of another manufacture. Further, these "interchangeable" cutouts are widely distributed throughout electrical distribution systems in this country, and large numbers of these cutouts are presently in service.
As mentioned earlier, after a fuse has operated to clear a fault, it must physically be replaced. This requires that utility personnel first locate which fuse or fuses have operated. Because of the complexities of modern networks, this is frequently a time consuming process, and is particularly difficult during adverse weather conditions.
Because of the need for a quick and simple means for detecting which fuses have operated, a particularly convenient and desirable fusing device known as the "dropout" fuseholder was created. A dropout fuseholder not only serves the primary function of protecting costly equipment from potentially damaging fault currents, but it also functions to provide a clear visual indication that the fuse has operated as an aid to utility personnel searching for a cause for an outage.
The dropout fuseholder typically includes a pair of terminals for connecting the fuseholder into a cutout mounting that is installed in the circuit that is to be protected, and an actuation means for causing the fuseholder to physically drop out of engagement with one terminal of the cutout mounting after the fuse has operated. The dropout fuseholder is designed such that, upon actuation of the fuse, one end of the fuseholder becomes disengaged from the cutout mounting. When this occurs, the unrestrained end of the fuseholder rotates down and away from its normal bridging position between the mounting gate, while the fuseholder remains supported from the mounting by its still-engaged end. The drop open feature provides the additional benefit of removing the fuseholder from the voltage stress otherwise associated with the energized conductor, eliminating the possibility of tracking and ultimate flash over around the fuse.
A typical prior art dropout fuseholder is disclosed in U.S. Pat. No. 3,611,240 (Mikulecky). As disclosed in that patent, upon actuation of the fuse, an explosive charge is ignited which actuates the dropout mechanism and frees the fuseholder to drop out of engagement with the cutout mounting in which it is installed. Similarly, U.S. Pat. No. 3,825,871 (Blewitt) also discloses a dropout fuseholder which employs an explosive charge to initiate dropout.
Although the drop open ability of the dropout fuseholder is a significant feature, the fuseholder must be carefully designed and applied to ensure that the fuse has sufficient time to clear the fault before dropout occurs. If the fuse has not operated to clear the fault by the time that dropout begins, an arc will be drawn between the stationary contact of the energized network and the now disengaged terminal on the fuseholder as it drops out and away from the stationary contact. The drawing of such an arc is undesirable for several reasons. First, the heat of the arc may damage the cutout terminals, making repairs necessary or requiring the replacement of the terminals, in addition to the fuseholder. Further, a long arc created as the fuseholder drops further away from the energized terminal may damage other nearby equipment, including the very equipment the fuse was intended to protect. Finally, the arcing creates the hazard of possible flash over around the fuse, meaning that the fuse will not be able to clear the fault. In this event, protective devices closer to the source must operate to clear the fault, possibly causing inconvenient and costly outages over portions of the network that would otherwise not have been affected if the fuse had functioned properly.
One means designed to retard premature dropout in a fuseholder is shown in Mikulecky '240. As best shown in FIG. 4 of Mikulecky, a dashpot device was employed between two members of the hinge assembly so as to oppose relative movement between these members, thereby delaying the drop open action. This feature has been successfully used for a number of years in the particular fuseholder disclosed in Mikulecky. However, not all fuseholders employ the arrangement of structural members that would accommodate the dashpot device of Mikulecky.
Accordingly, despite advances made in fuse technology over the years, further improvements would be welcomed by the industry. Specifically, there is a need for a dropout mechanism that would retain the fuseholder in its current-carrying position until after the fuse has had sufficient time to clear the fault before dropout is initiated. Ideally, the device would be simple and inexpensive to manufacture and would provide dependable operation in a variety of fuseholder application, and under widely varying ambient temperatures.